1. Field of the Invention
The present invention relates to a projection displaying apparatus (for example, a liquid crystal projector) using a reflection displaying device (for example, a reflection liquid crystal panel), and an optical system (i.e., an optical engine) usable in a projection displaying apparatus. Particularly, the present invention relates to techniques of cooling a polarization plate.
2. Background of the Invention
As a conventional technology, Japanese Patent Application Laid-Open Nos. H9-288315 and H10-197954 disclose a liquid crystal projector using a transmission displaying device (a transmission liquid crystal panel), in which a polarization plate is disposed on an optical path between a condenser lens and the transmission liquid crystal panel, being away from each of them. More specifically, the polarization plate disclosed in the Japanese Patent Application Laid-Open No. H10-197954 is comprised of a transparent substrate and a film-like polarization element bonded to the transparent substrate.
A polarization plate absorbs one of a P-polarized light component and an S-polarized light component of light from a light source, and transmits the other one only. In the polarization plate, accordingly, the absorbed polarized light component is converted into a thermal component to generate heat. A method of cooling the heat generated in the polarization plate by a cooling fan is known.
In a cooling method disclosed in the Japanese Patent Application Laid-Open No. H10-197954, a cooling wind from a cooling fan disposed in a lower portion is passed through a space between the polarization plate and the transmission liquid crystal panel (namely, the space is used as a wind guiding path for guiding the wind from the cooling fan) such that the polarization plate and the transmission liquid crystal panel can be cooled. Further, there is a disclosure in the Japanese Patent Application Laid-Open No. H10-197954 that a wind-direction guiding portion is provided in the space between the polarization plate and the transmission liquid crystal panel, and the cooling wind from the cooling fan disposed in the lower portion is guided by the wind-direction guiding portion to be blown on a surface of the polarization plate (further a surface of the transmission liquid crystal panel), so that the polarization plate can be effectively cooled. Furthermore, there is a disclosure in the Japanese Patent Application Laid-Open No. H10-197954 that the cooling fan is disposed slantingly relative to the surface of the polarization plate, and the cooling wind from the cooling fan is thereby blown directly on the surface of the transmission liquid crystal panel and the surface of the polarization plate, so that the polarization plate can be effectively cooled.
In the Japanese Patent Application Laid-Open No. H9-288315, there is a disclosure that a wind guiding plate is disposed in a space between the polarization plate and the transmission liquid crystal panel, and the polarization plate is arranged a little slantingly relative to a surface of the transmission liquid crystal panel, so that the polarization plate can be effectively cooled.
On the other hand, Japanese Patent Application Laid-Open No. 2001-154268 discloses a liquid crystal projector using a reflection displaying device (a reflection liquid crystal panel).
FIG. 8 illustrates the structure of a conventional liquid crystal projector using a reflection liquid crystal panel. In FIG. 8, reference numeral 58 denotes a dichroic mirror for transmitting therethrough light in a green (G) wavelength range (light in a first wavelength range), and reflecting light in red (R) and blue (B) wavelength ranges (light in a second wavelength range), out of white light emitted from a light source 51. A light-incident surface of the dichroic mirror 58 is set slantingly relative to the white light incident thereon at an angle of about 45 degrees.
Reference numeral 60 denotes a first polarization beam splitter with a polarization splitting face for transmitting therethrough a P-polarized light component, and reflecting an S-polarized light component. The polarization splitting face of the first polarization beam splitter 60 is interposed on an optical path of light in the first wavelength range between the dichroic mirror 58 and a green reflection liquid crystal panel 61G. Reference numeral 66 denotes a second polarization beam splitter with a polarization splitting face for transmitting therethrough a P-polarized light component, and reflecting an S-polarized light component. The polarization splitting face of the second polarization beam splitter 66 is interposed on an optical path of light in the second wavelength range between the dichroic mirror 58, and a red reflection liquid crystal panel 61R and a blue reflection liquid crystal panel 61B. On an incident side of the second polarization beam splitter 66, bonded is a color selective phase difference plate 65 that has a function of changing by 90 degrees a polarization direction of light in a blue (B) wavelength range only out of light in the second wavelength range.
Reference numeral 59 denotes a first incident-side polarization plate interposed on the optical path of light in the first wavelength range between the dichroic mirror 58 and the first polarization beam splitter 60. Further, reference numeral 64 denotes a second incident-side polarization plate interposed on the optical path of light in the second wavelength range between the dichroic mirror 58 and the second polarization beam splitter 66 (the color selective phase difference plate 65).
In the Japanese Patent Application Laid-Open No. 2001-154268, there is a disclosure that a space between the first incident-side polarization plate 59 and the first polarization beam splitter 60 and a space between the second incident-side polarization plate 64 and the second polarization beam splitter 66 (the color selective phase difference plate 65) are used as a wind guiding path for the cooling fan such that the polarization plate and the polarization beam splitter can be cooled.
A cooling method disclosed in the Japanese Patent Application Laid-Open No. H10-197954, however, has a problem that the cooling efficiency is not so high, because the polarization plate is gradually cooled from its lower portion to its overall plate by the cooling wind from the cooling fan provided in a lower portion. Further, though the cooling wind is blown on the surface of the polarization plate by the wind-direction guiding portion interposed in the space between the polarization plate and the transmission liquid crystal panel, a space for the wind-direction guiding portion is needed between the polarization plate and the transmission liquid crystal panel, leading to an increase in the size of an optical system. Furthermore, though the cooling wind is also blown on the surface of the polarization plate by arranging the cooling fan slantingly relative to the surface of the polarization plate, a space for a cooling portion is expanded due to the slant arrangement of the cooling fan, leading to an increase in the overall size of the apparatus.
Further, the cooling method disclosed in the Japanese Patent Application Laid-Open No. H9-288315 also has a problem that a space for the wind guiding plate is needed in a space between the polarization plate and the transmission liquid crystal panel, and the space is expanded due to the slant arrangement of the polarization plate, leading to an increase in the size of an optical system.
In the liquid crystal projector using the reflection liquid crystal panel disclosed in the Japanese Patent Application Laid-Open No. 2001-154268, large spaces (triangular spaces due to the 45-degree inclination of the dichroic mirror 58) are formed between the dichroic mirror 58 and the incident-side polarization plates 59 and 64, respectively, as illustrated in FIG. 8.